Currently, steam and gas turbine manufacturers use a variety of hydraulic actuators to control the position of a turbine's many control and shutoff valves. These actuators are typically coupled directly to the valve and can completely shut off the flow of fuel/steam into the turbine in less than 250 milliseconds. As with nearly all hydraulic systems, the single biggest drawback of hydraulic actuation is its high susceptibility to failures due to hydraulic fluid contamination.
Furthermore, hydraulic actuation systems require hydraulic fluid (typically oil) to be in close proximity to the turbine casing. Since steam turbine casings can often be heated to temperatures in excess of 1000° F. during operation, any hydraulic fluid leaks pose an immediate fire danger to the turbine operators and equipment.
In response to these problems, the assignee of the instant application has started providing many different electric actuation solutions to turbine manufacturers. Historically, electric actuators have not been used due to slow slew speeds and reliability issues associated with electronics exposed to high temperatures. Technology advancements in electrical components and thermal design considerations have allowed for electronic products to be reliably exposed to high temperatures, but slow slew speeds remains a problem for electric actuation.
Electric actuators are composed of four main components: an electric motor; gearbox; lead screw; and return spring. Past solutions have attempted to achieve a high speed trip function by adding an electromagnetic clutch between the gearbox and lead screw (or at an intermediate stage in the gearbox). When the clutch is engaged, the lead screw is coupled to the gearbox and actuation is possible. When de-energized, the lead screw is de-coupled from the gearbox and allowed to freely spin as the actuator return spring moves the actuator to the fail-safe position. De-coupling the lead screw from the gearbox also reduces the amount of inertia attached to the screw and allows the device to accelerate quickly.
The problem with using an electromagnetic clutch comes in its size-to-torque capability and speed limitations. If a clutch is used between the lead screw and gearbox, it must have a very high torque capability. As torque capability increases, so too does the physical size of the clutch and the solenoid used to actuate it. As the solenoid grows, the time it takes to de-energize also increases to often unacceptably slow release times.
In view of this problem alternate designs position or move the clutch to an intermediate stage of the gearbox which will reduce the torque needed by the clutch. However, this increases the speed at which the clutch must spin. Regardless of where the clutch is positioned in the power train, the power (speed*torque) it must transmit remains the same. This will often result in speeds in excess of 10,000 RPM which greatly exceeds the speed limitations of most commercially available clutches. This also increases the inertia attached to the roller screw, slowing the acceleration of the screw during a trip to often unacceptably slow release times.
What is needed is an electric actuator that has high speed, fail-safe functionality that can be independently triggered in the event of an emergency shutdown. The invention provides such an electric actuator. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.